This invention relates to electron gun emission testing equipment for color cathode ray tubes.
Properly operating electron guns of a color cathode ray tube should operate at adequate individual and relative emission levels. The electron guns must operate at proper relative emission levels to ensure a proper gray scale for a black and white picture producing video signal which would otherwise produce a colored tint if the relative emission strengths of the electron guns do not fall within an acceptable range of values. To this end the ratio between the highest and lowest emitting electron gun of a color cathode ray tube under voltage conditions producing a bright white indication must not be greater than 1.5 to pass the relative emission test under present standards.
In carrying out an emission test, commonly a fixed predetermined negative bias voltage is first applied between each control electrode and associated cathode electrode and an adjustable positive voltage is applied between each accelerator electrode and the associated cathode electrode and adjusted to a value so current flow between the cathode and accelerator electrodes indicated on a current meter just begins to flow, to produce what is called a spot cutoff emission level. The various electron guns generally require different positive voltage levels between the accelerator and cathode electrodes to produce a spot emission cutoff level. (Negative voltages applied between the control and cathode electrodes can be the individually adjustable spot emission cutoff obtaining voltages, but this is not a convenient way to test the emission current.) The voltage on each control electrode is then switched to a value like chassis ground, which does not affect the previously adjusted accelerator to cathode voltage, to provide a high emission level which would produce a bright white intensity on the face of the cathode ray tube under normal operating conditions. (Under emission test conditions, there is no voltage applied to the screen of the cathode ray tube.) The control and accelerator electrode voltages are generally fed to the electron guns by manually operable switches which feed the voltages involved to the electron guns in a sequence selected by the operator. The operator notes the relative maximum emission values during the various measurements and computes the ratio of the emission levels of the highest and lowest emitting guns to see if the ratio exceeds an acceptable 1.5 to 1 value. In some emission testing equipment, a separate current meter is provided for each electron gun so that the emission of all of the electron guns can be simultaneously measured and indicated, making it an easier matter to determine the emission ratio of the electron guns. However, it is extremely difficult to design emission testing equipment which provides for simultaneous emission of all of the electron guns of all types of cathode ray tubes, including the recently developed "in-line" cathode ray tubes which have commonly connected control and/or accelerator electrodes. The difficulty is caused by the problem of isolating the spot emission cutoff level voltage adjusting circuits connected between commonly connected accelerator electrodes and the individual cathodes of the electron guns from the circuit which applies the initial negative reference voltage and the high intensity emission producing voltage between commonly connected control electrodes and the cathodes. (Obviously, the switching of the commonly connected control electrodes from an initial reference level to a high intensity producing level should not effect the voltages coupled between the commonly connected accelerator electrodes and the individual cathode electrodes.)
In prior emission testing techniques which simultaneously measure the emission of all of the electron guns, the simultaneous flow of emission current from the three electron guns require relatively expensive well regulated power supplies to avoid substantial variations of the voltages fed to the accelerator electrodes when the control electrode voltage is switched to a bright white intensity producing value. Another disadvantage of prior emission measuring techniques in using sequential manual switching is the accompanying time delay involved for making the test, including the waiting time for permitting the emission of an electron gun to stabilize. Furthermore, less than desirable accuracy is achieved in obtaining relative emission measurements where the electron guns are made successively operable by manual switching techniques, since the separately operating electron guns do not duplicate or nearly approximate actual operating conditions and interactions between operating electron guns and the possibility of intermittent operating conditions are not then taken into account.